Question

Show all calculation to receive credit for an answer. Be sure to indicate your final answer where appropriate L 1. An amine complex of cobalt(III) is yellow-orange and has a beau of 475 nm. The standard curve below is for a set of standards measured at ay in a 1.8 cm cell. Use it to answer the questions that follow. 0.3 0.7 0.6 0.5 Absorbance 04 0.3 0.2 01 0 0 0.05 0.1 015 02 Concentration Com 0.25 0.3 0.35 a Determine the value for the average molar absorptivity coefficient. 6. Suppose you ran the same samples in a 1 cm cell Explain, if any, changes in absorbance, concentration, and molar absorptivity:

Answer 1

Solution:

Given data:

a) Since, Absorption, A = ε × c × l

where, ε is the molar absorptivity coefficient.

c = concentration of Co(III) complex solution.

l = path length of the cell holder.

Hence, ε = A/cl

From the calibration plot, Average molar absorptivity,

ε = Slope/l ; where slope = A /c

Now from plot, Slope = (Y₂―Y₁)/(X₂―X₁)=(0.28 - 0.12)÷(0.1 - 0.05) = 3.2

Now path length , l = 1.8 cm

Therefore, ε = (3.2÷ 1.8) = 1.78 L mol^-1 cm^-1.

b) Since, ε = A / (c × l), so with decreasing path length(l) , the molar absorptivity coefficient (ε) increases.

i.e., at l = 1.2, ε = (3.2 ÷ 1.2) = 2.67 L mol^-1 cm^-1.

Hence , molar absorptivity coefficient increases as path length decreases.

c) Since, the molar absorptivity coefficient depends on wavelength so from absorbance vs. Wavelength plot we cane to know that if we measure the absorbance of the same standards above the λmax then the nature of the abs. Vs conc. graph will change and will not show linearity.

d)  Absorption refers to a physical process of absorbing light, while absorbance does not always measure the Absorption; it measures the reduction of transmitted radiant power. It can be caused by Absorption also, but also reflection, scattering, and other physical processes.

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